1. Field of the Invention
The present invention relates to a feedback control device and also relates to a control device performing feedback control of a motor of various mechanical devices.
2. Description of Related Art
Conventionally, an electric motor is used as a driving source of various mechanical devices. Particularly, operation at high accuracy is required while ensuring sufficient driving force for a measuring apparatus, a machine tool, and the like. To meet such a requirement, a feedback control device having triple control loops (position loop, speed loop, and electric current loop (torque loop)) is used as a control device controlling the motor (see Japanese Patent Laid-open Publication No. 2013-021804).
Of these loops, the electric current loop may be subjected to performance restriction by an electric current sensor for feeding back. For example, a sensor detecting micro currents at high accuracy experiences difficulty in securing a wide detection range extending from micro currents to large currents. On the other hand, a sensor detecting large currents experiences difficulty in detecting electric currents with a high degree of accuracy. In order to eliminate such restrictions, a method has been suggested in which a plurality of electric current detection systems are used on the electric current loop (see Japanese Patent Laid-open Publication No. 2014-183609).
In Japanese Patent Laid-open Publication No. 2014-183609, a sensor capable of measuring over a wide range covering a large current is used and its output is divided into two systems and an amplifier is provided to a first system. For large currents, sensor output obtained through a system without the amplifier is used as is, while for micro currents, the sensor output is amplified by the amplifier to obtain high accuracy.
In a case where the plurality of electric current detection systems mentioned in Japanese Patent Laid-open Publication No. 2014-183609 are used by switching between the systems, a difference in the signals output by each system may arise. When the signal output difference is generated by each system, an electric current value that is fed back changes abruptly due to switching between the plurality of electric current detection systems, and therefore concerns may arise such as an excessive flow of the electric current and instability of the electric current loop. Accordingly, the output difference which accompanies switching between the electric current detection systems is preferably kept to a minimum.
This has been addressed by Japanese Patent Laid-open Publication No. 2014-183609 such that each system is preferably provided with an equivalent or common sensor (paragraph [0007] of Japanese Patent Laid-open Publication No. 2014-183609). However, even with a sensor having identical characteristics, a difference may arise due to changes over time. In addition, even with a common sensor, an output difference may be generated by having the amplifier.
Further, there may be a desire for selectively using a sensor for the large current and a sensor for the micro current. For such different sensors, in order to eliminate the output differences of each, a case may be considered where an amplifier is provided to adjust each output. However, even with such techniques, the amplifier adjusting each output may experience changes over time. Therefore, a configuration is desired which is capable of preventing or reducing a dramatic change in the detection value at the time of switching between the detection systems.
These concerns are not limited to feeding back the electric current loop, but may also be concerns for feeding back the speed loop and the position loop. Particularly, in the electric current loop, an excessive flow of the electric current is a serious concern for circuits and wiring.